The invention relates to magnetic storage media, and in particular, to erasing of magnetic tape media.
Magnetic tape media is often used for storage and retrieval of data, and comes in many widths and lengths. Magnetic tape media remains an economical medium for storing large amounts of data. For example, magnetic tape cartridges, or large spools of magnetic tape are used to back up large amounts of data for large computing centers. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or laptop computers.
During the lifespan of magnetic tape, it may be necessary or desirable to erase the tape. For example, an erasing process is typically implemented during the fabrication of magnetic tape media. Typically, magnetic tape is erased after being coated, cut and spooled, and then servo tracks are written on the magnetic tape after it has been erased. At this stage, there typically is no data stored on the tape. Nevertheless, it is desirable to perform erasure prior to servo writing to ensure that the servo patterns can be properly written.
In addition, magnetic tape may also be erased later in the lifespan of the magnetic tape. For example, the tape may be erased after being recorded, thus putting the tape into a condition to be re-recorded. In this document, the term xe2x80x9cerasurexe2x80x9d refers to a process or an apparatus that returns the magnetic tape or a portion thereof to a substantially demagnetized condition, i.e., a condition in which the fields of the individual magnetic particles on the tape substantially cancel out. Often the demagnetized condition is one in which the individual magnetic particles have substantially no ordered magnetization pattern.
Erasure is typically accomplished by subjecting the magnetic tape to a magnetic field of sufficient magnitude to saturate the magnetic particles on the tape. The polarity of the applied field is then reversed and the magnitude of the applied field is reduced by some small amount. The polarity of the applied field is then reversed again and the magnitude is again reduced. The process of reducing the magnitude of the applied field and reversing the polarity of the applied field continues until the magnitude reaches zero, or some finite value which is determined to be sufficiently small as to have no further effect.
One common method of erasing involves subjecting an entire reel of tape to a field which is generated by an alternating electric current. The alternating current produces the reversal of magnetic field, and the decay in field strength is provided either by reducing the magnitude of the applied current or by physically removing the media from the erasing field.
Another method of erasing implements a specially designed recording head driven with alternating current. As the tape passes by the head, it is erased by the alternating field generated by the recording head. In order to leave the media in a demagnetized state, each point of the media is subjected to several reversals of field while passing by the head. The frequency of the erasing field is sufficiently high to produce the required number of reversals. The strength of the field, as seen by the tape, decays as the tape moves away from the head.
These conventional methods of erasing magnetic tape, however, are relatively expensive, requiring specially designed degaussing chambers or recording heads. Moreover, these conventional methods can be unreliable. For example, if the proper polarity and intensity of the alternating magnetic field is not maintained, the magnetic tape may still be magnetized following a conventional erasing process.
In general, the invention is directed to methods and apparatus for erasing magnetic tape. The tape to be erased is fed through an erasure unit that includes an array of discrete magnetic elements. The array of magnetic elements includes a first set of discrete magnetic elements and a second set of discrete magnetic elements. The magnetic fields for the respective first and second sets of magnetic elements have opposite polarity, as seen by the tape as it feeds through the erasure unit.
The magnetic elements in the first and second sets are positioned to define an alternating configuration. For example, each magnetic element in the first set may be positioned proximate one or more elements in the second set, and likewise, each magnetic element in the second set may be positioned proximate one or more elements in the first set. Thus, as the tape feeds through the erasure unit, it encounters magnetic fields that alternate in polarity. In addition, the strength of the fields, as seen by the tape as it passes through the erasure unit, decrease. For example, either the signal strength of the magnetic elements used, the distance between the tape and the elements, or both can be controlled to ensure that the tape encounters alternating fields that decrease in strength. Exponentially decreasing signal strength is preferred.
The discrete magnetic elements (having opposite polarity) may be separated by polar elements. For example, each magnetic element may reside in a gap between two polar elements. The polar elements may act as magnetic poles between proximate magnetic elements. In addition, the polar elements can be formed to focus the magnetic fields, thereby increasing the field strength seen by the tape as the tape passes by a given gap. In addition, non-magnetic material, such as a urethane may fill the gaps to avoid the collection of debris on the magnetic elements.
The erasure unit according to the invention can provide several advantages over conventional methods for erasure. For example, although the invention is not limited in this respect, the erasure unit can use passive magnetic elements, thus requiring no external power supply and therefore avoiding any electronic malfunction. In addition, the erasure unit can be fabricated at relatively low cost. Moreover, the erasure unit can be incorporated into other manufacturing machines or processes, thereby reducing the amount of time and the costs associated with magnetic tape manufacturing. For example, the erasure unit could be incorporated into a tape coating system, a tape cutting system, or a servo writing system. Finally, the erasure unit can be made removable and/or created to have a bypass tape path. In this manner, a given tape manufacturing device or process that implements the erasure unit can operate with or without erasing the tape.
The details of various embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present invention will be apparent from the description and drawings, and from the claims.